1. Field of the Invention
The present invention relates to a current driving type emissive display apparatus such as an organic electro luminescence (hereinafter referred to as an “organic EL”) device having a thin-film transistor and a method for driving the apparatus, and more particularly, to a technique for realizing a reduction in deterioration over time or a reduction in both deterioration over time and electric power consumption simultaneously.
2. Description of Related Art
An operation of a conventional current driving type emissive apparatus such as an organic EL having thin-film transistors will be described with reference to FIGS. 16, 17, and 18.
FIG. 16 is an equivalent circuit diagram of one pixel of the conventional organic EL display device having thin-film transistors, FIG. 17 is an equivalent circuit diagram showing a matrix construction of the conventional organic EL display device having thin-film transistors, and FIG. 18 is a driving voltage diagram of the conventional organic EL display device having thin-film transistors.
Provided are: a data line 112; a first switching element 121 (hereinafter referred to as a “switching thin-film transistor”) in which the source terminal side is connected to the data line 112 and a gate electrode is connected to a scanning line 111; a holding electrode 113 for storage capacity, in which one terminal is connected to the drain terminal side of the switching thin-film transistor 121; a second switching element 122 (hereinafter referred to as a “current thin-film transistor”), in which a gate terminal is connected to the drain terminal of the switching thin-film transistor and the source terminal is connected to a first feeder 114; and an organic EL device 135, one terminal of which is connected to the drain terminal of the current thin-film transistor and the other terminal of which is connected to a second feeder.
The switching thin-film transistor 121 controls the conduction between the data line 112 and the holding electrode 113 by the potential of the scanning line 111. That is, a scanning potential 211 controls the conduction between a signal potential 212 and a holding potential 213. Although an n-channel type thin-film transistor is employed as the switching thin-film transistor 121 in this example, a p-channel type thin-film transistor may also be employed. In such an example the high-potential-side of the scanning potential 211 and the low-potential-side thereof are reverse to those of this embodiment.
In a pixel in a displaying condition, the signal potential 212 is high and the high potential is held at the holding potential 213. In a pixel in a non-displaying condition, the signal potential 212 is low and the low potential is held at the holding potential 213.
The current thin-film transistor 122 controls the conduction between the first feeder 114 and a pixel electrode 115 by the potential of the holding electrode 113. That is, the holding potential 213 controls the conduction between a first feed potential 214 and a pixel potential 215. Although an n-channel type thin-film transistor is employed as the current thin-film transistor 122 in this example, a p-channel type thin-film transistor may also be employed. In such an example the high-potential-side of the signal potential 212 and the low potential thereof are reverse to those of this embodiment.
In a pixel in a displaying condition, the holding potential 213 is high, so the first feeder 114 and the pixel electrode 115 are electrically connected. In a pixel in a non-displaying condition, the holding potential 213 is low, so the conduction between the first feeder 114 and the pixel electrode 115 is interrupted.
In a pixel in a displaying condition, the current flows from the first feeder 114 through the current thin-film transistor 122 and the pixel electrode 115 to a second feeder 116, and the organic EL device 135 emits light. In a pixel in a non-displaying condition, no current flows, and the organic EL device will not emit light.
Since the first feed potential 214 is higher than a second feed potential 216, the current flows from the first feeder 114, through the current thin-film transistor 122, the pixel electrode 115, and the organic EL device 135 to the second feeder 116.
The actual operation of organic EL display apparatuses with thin-film transistors is not as simple as that described above and the devices operate under more complex relationships of voltages and currents. Similarly and qualitatively, however, the above description holds true.
FIG. 19 is a sectional view of the organic EL display apparatus having a conventional thin-film transistor. FIG. 20 is a plan view of the organic EL display apparatus having the conventional thin-film transistor. A section taken along the line A—A of FIG. 19 corresponds to a section taken along the line A-A′ of FIG. 20.
In the organic EL device 135, a current flows from a high-electric-potential-side electrode 165 of the organic EL device through a luminescent material 155 for the organic EL device to a low-electric-potential electrode 175 of the organic EL device. Although PPV, ITO, and Al are respectively employed as the luminescent material for the organic EL device 155, the material for the high electric potential side electrode 165 of the organic EL device, and the material for the low-electric-potential electrode 175 of the organic EL device in this example, other materials may also be used.
In the conventional example, in which an AC voltage is applied between the source terminal and the drain terminal of the switching thin-film transistor 121, to cause an alternating current to flow, a DC voltage is applied between the source terminal and the drain terminal of the current thin-film transistor 122 to disadvantageously cause a direct current to flow. This is due to an asymmetric configuration optimizing the materials of the high and low-potential-side in order to improve the luminous efficiency of the organic EL device 135; the organic EL device emits light because of the DC voltage being applied causing the direct current to flow. However, when a DC voltage is applied not only to the organic EL device but also to the thin-film transistor or a direct current flows, a rapid deterioration of the thin-film transistor over time will be caused.
On the other hand, an AC voltage may also be applied between the source terminal and the drain terminal of the current thin-film transistor 122. In this case, an alternating current will not flow through the organic EL device 135, but only a one-way current flow, because of the rectification of the organic EL device 135. In other words, the organic EL device 135 emits light in one direction, while it does not emit light in the other direction, so that the luminous efficiency deteriorates. Thus it requires an increase in power consumption to obtain the same amount of light emission as in the case in which a DC voltage is applied to cause a direct current to flow.